Berzhiani and Nesti re-examined an experiment at Grenoble with trapped a half million neutrons in a trap of volume 190, within which the earth magnetic field was screen out, the result approximately 5.2 sigma signal for neutron oscillating into something else of the same mass, with a period of between 2 and 10s. The mirror oscillating result requires the Earth as a mirror magnetic field as well as its normal magnetic field which might happen if the Earth had trap a significant amount of mirror matter. The oscillation is strongest with the normal and mirror magnetic field are nearly the same, in this case around 0.2 Gauss, and a time period of around 3 seconds.
Previous experiments have looked for mirror oscillations in orthopositronium (an electron orbiting with a positron with opposed spins), and found nothing. Mirror Matter theory itself does not require neutrons to oscillating into mirror neutrons with any particular speed, so this was not an expected result. If the experiment is confirmed, it would be the first strong evidence of the Mirror Matter scenario.
Mirror Matter is a particularly interesting form of dark matter, because unlike WIMP dark matter, it might be useful technologically. It also forms stars which though invisible would help explain why dark matter does not seem to clump together as strongly as predicted in by standard WIMP dark matter.